University of California
Herbicide Symptoms

Synthetic Auxins

HRAC Group: O

WSSA Group: 4, 19

Synthetic Auxins and Auxin Transport Inhibitors are generally used for controlling broadleaf weeds in grass crops, pastures, and industry. These herbicides include some of the more effective chemicals for perennial broadleaf weed and brush control. These herbicides, also known as plant growth regulators, are readily absorbed through both roots and foliage and translocate by phloem or xylem to meristematic tissue interfering with cell formation that results in abnormal root and shoot growth. The killing action of synthetic auxins is not caused by any single factor but rather by the disruption of several growth processes in susceptible plants. It seems, however, that the primary action of these herbicides is likely to affect cell wall plasticity and nucleic acid metabolism. Synthetic auxins also affect protein synthesis, cell division and growth, and stimulate ethylene evolution, which may in some cases produce the characteristic epinastic symptoms associated with exposure to these herbicides.

The synthetic auxins include the following herbicide families:  benzoic acids, phenoxycarboxylic acids, pyridine carboxylic acids, and quinoline carboxylic acids that act similar to that of endogenous plant auxin. Auxin Transport Inhibitors such as diflufenzopyr, however, inhibit the movement of auxinic compounds out of cells. Consequently, when combined with a synthetic auxin herbicide such as dicamba, the dicamba can move into the cells but cannot move back out of the cell, thus maintaining a greater concentration of the auxinic herbicide within the cell. Diflufenzopyr has minor herbicide activity when applied alone but enhances the activity of auxinic herbicides.

Injury Symptoms: Most auxin herbicides cause similar injury symptoms, but symptom intensity and appearance depend on the herbicide, level of exposure, growth stage, crops, and environmental conditions. In general, trees and shrubs are less sensitive than susceptible annual plants. In addition, younger plants are more susceptible to Auxin Inhibitors than mature plants. Symptoms may range from slight, at low exposure, to severe or death from high levels of exposure. Initial symptoms are twisting, leaf curling, and cupping, which may occur within hours of exposure. Leaves that are not fully expanded at the time of exposure may be stunted and distorted. A few days after exposure, general chlorosis may develop at high exposure levels. Leaves will drop and shoot tips may die, followed by stem dieback in trees and vines. Growth may resume depending on the level of exposure. Regrowth is sometimes limited to buds on the lower part of the plants. Regrowth may exhibit severe shoot and petiole twisting, leaf cupping, stunting, curling, strapping, feathering, roughness, crinkling of the leaf surface, vein discoloration, and fingering of the leaf margins. Auxin herbicides may inhibit interveinal tissue growth making veins appear to be joined together and extended to form finger-like projections. In trees, ornamental shrubs, and grapevines, symptoms may continue to appear until the end of the growing season. In grapevines, 2,4-D symptoms from high concentrations may continue to appear in the second year after exposure. Severely injured grapevines may not recover for two years or more. Furthermore, high concentrations of Auxin Inhibitors may cause stem cracking or dark reddish coloring. Stem elongation of plants may be enhanced (at low concentrations) or inhibited (at high concentrations) by growth Auxin Inhibitors.

Injury from high concentrations of Auxin Inhibitors may change fruit size, shape, and appearance or cause abortion of fruits. Slight auxin herbicide symptoms, however, may have no effect on fruit maturity. Exposure to high concentrations of auxin herbicides may also delay fruit ripening when plants are severely affected. For example, delayed maturity from exposure to high 2,4-D concentrations may exist in a grapevine for one to three years before normal ripening returns.

Chemistry Group and Common Names of Synthetic Auxins
Used in the United States

Chemistry Common Name
Phenoxycarboxylic acids 2,4-D
  2, 4-DB
Benzoic acids Dicamba
Pyridine carboxylic acids Clopyralid
Quinoline carboxylic acids Quinclorac
Semi-carbazones Diflufenzopyr
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